Exemplary devices are found in automated production- and testing systems where balances of a modular configuration—so-called weighing modules—are particularly well suited to be integrally incorporated into these systems. In essence, the balances used for this purpose are of the type where the display unit is arranged separately from the balance, for example in a system with a central display unit for a plurality of weighing modules. Integrated weighing modules of this kind are used in systems for the production and testing of small and relatively expensive parts, for example in filling- and packaging machines for tablets, capsules, ampoules, etc. in the pharmaceutical industry, or in the checking of ball bearings. The weighing of objects of the same kind, or also the so-called batch-weighing, is a process in which a plurality of loads need to be weighed individually, be it for the purpose of checking, dosage-dispensing, filling, or other applications, within a confined space.
Since a handling device such as a robotic arm with multiple grippers is used to put the weighing objects onto the individual load receivers of the weighing modules and to remove them after they have been weighed, the positions of the individual load receivers in relation to each other and in relation to the handling device have to be accurately and durably set.
Devices of this kind which are used for weighing objects of a uniform kind belong to the known state of the art. Predominantly, these devices are arrangements of weighing modules in a row or a two-dimensional array. Other arrangements are based on the concept of placing the weighing modules in a satellite-like arrangement around a serial line-up of load receivers which have to be matched to the distances between the feeder elements of an existing handling device, because the weighing module is often too large to allow an arrangement at the required close intervals.
A serial line-up of weighing modules is disclosed in DE 102 42 118 A1 and DE 199 20 494 A1, wherein four weighing modules are arranged in a row in an apparatus for weighing pharmaceutical receptacles, in particular ampoules, wherein before and after the filling the receptacles are brought to and removed from the weighing modules by a gripper device.
A two-dimensional arrangement of weighing cells is disclosed in JP 01212327 A, which describes a cost-effective method of producing a large number of weighing cells from a plate of spring material to which strain gauges are bonded as sensor elements. However, in contrast to a weighing cell that works according to the principle of electromagnetic force compensation, these weighing cells which work with strain gauges are not suitable for the area of application where masses in the range from micrograms to grams have to be determined.
In a weighing cell that functions according to the principle of electromagnetic force compensation, the force that is caused by a load on the weighing pan is compensated by a force-compensating member consisting of a permanent magnet and a coil, wherein the current is measured which flows through the coil to generate the compensating force. The measured value is in proportion to the load placed on the weighing pan. However, the measured value is also dependent on the position of the coil in the magnetic field of the permanent magnet and therefore, when determining the measurement value, the coil always has to have the same position in relation to the magnet. The position of the coil after applying the load is determined by way of a position sensor, and the current through the coil is increased until the load-related displacement of the coil in relation to the permanent magnet is compensated. At this point the coil current is measured, which represents a measure for the weight of the applied load. A weighing cell of this type is disclosed in CH 638 894 A5, wherein the weighing cell has a force-transmitting device which is arranged between the load receiver and the force-compensating member and which transmits the force generated by the load on the load receiver to the force-compensating member, reducing or magnifying the force depending on the load range.
A balance that works according to the same principle is disclosed in CH 593 481 A5. In this patent, the load receiver is coupled directly to the force-compensating member by way of a force-transmitting rod. The movable part of the position sensor is attached to the force-transmitting rod, while the stationary part of the position sensor is rigidly connected to the housing-based part of the weighing cell, or generally to the stationary part of the force-compensating member. This arrangement which is referred to as direct measuring principle is used in the range of small loads. As the position sensor has only a limited resolution, the precision of the measurement depends essentially on the resolution of the position sensor.
The load receiver and the coil of the force-compensating device have to be precisely guided in relation to the stationary part of the weighing cell. This is accomplished by a parallel-guiding mechanism whose movable parallelogram leg is connected to the force-transmitting rod and whose stationary portion is rigidly connected to the housing-based part of the weighing cell. The movable parallelogram leg and the stationary portion are connected to each other through two parallel-guiding members that are rigid against bending and have thin flexure joints. However, one could also use spring-like parallel-guiding members, in which case the thin flexure joints are omitted. When a load is placed on the load receiver, the force-transmitting rod moves in the direction of the load, whereby the parallel-guiding members are displaced and the thin flexure joints or spring-like elastic guide members are caused to bend. Analogous to a leaf spring element, these thin flexure joints or spring-like elastic guide members generate a moment of a magnitude that is in proportion to the angle of deflection of the parallel-guiding members, acting in the opposite direction of the bend, or a force acting in the opposite direction of the load. The more massive the thin flexure joints are designed, the larger is the load differential that is needed to produce the minimally detectable displacement of the position sensor. Thus, the dimensions of the flexure joints or the elastically flexible parallel-guiding members also significantly influence the resolution of the weighing cell.
Parallel-guiding mechanisms which guide a weighing pan in vertical motion by an upper and a lower parallel-guiding member often have adjustment means at two of the connecting areas between the parallel-guiding member and the stationary part, whereby these connecting areas can be adjusted in a given direction. Position-adjustable connecting areas of this kind which allow the weighing modules to be adjusted for eccentric load errors are disclosed in DE 27 10 788 A1.
The parallel-guiding mechanisms disclosed in the prior art have a disadvantage that a maximum allowable stress for the material used can impose limits on how far the thickness of the flexible joints can be reduced, and that the thinning-down of the flexure joints or the elastically flexible parallel-guiding members makes the parallel-guiding mechanism very susceptible to being damaged by overloads. To mitigate this problem, the parallel-guiding members can be made longer. This leads to a smaller angle of deflection associated with the minimal detectable displacement of the position sensing device. However, as a result of this design measure, the weighing modules that are used in a device for weighing objects of a uniform kind will have very unfavorable dimensions which lead to expensive, voluminous and complicated arrangements.